1. Field of the Invention
The invention relates to an improved self-boosting electromechanical friction brake for a motor vehicle.
2. Description of the Prior Art
One friction brake of the type with which this invention is concerned is known from International Patent Disclosure WO 03/056204 A1. The known friction brake is embodied as a disk brake and has a friction brake lining, which for braking can be pressed by an electromechanical actuation device against a brake body to be braked; in the case of a disk brake, the brake body is a brake disk. The actuation device of the known friction brake has an electric motor, a step-down gear, and a screw drive as a rotation/translation conversion gear. With the actuation device, the friction brake lining can be moved transversely or at an angle obliquely to the brake disk and thus pressed against it. The construction of the actuation device can differ from the construction described here.
To attain self-boosting, the known friction brake has a ramp mechanism, with a ramp extending at an angle to the brake disk, on which ramp the friction brake lining is braced upon being pressed against the brake disk. If in braking the friction brake lining is pressed against the rotating brake disk, then the brake disk exerts a frictional force on the friction brake lining, which urges the friction brake lining in the direction of an increasingly narrow wedge-shaped gap between the ramp that supports the friction brake lining and the brake disk. The bracing of the friction brake lining on the ramp, extending obliquely to the brake disk, of the ramp mechanism exerts a force on the friction brake lining that has a force component transverse to the brake disk. This force component transverse to the brake disk is a contact pressure, which presses the friction brake lining against the brake disk. The contact pressure exerted by the ramp mechanism increases contact pressure exerted by the actuating device and thus increases a braking force of the friction brake. This increasing of the contact pressure and braking force is called self boosting.
An angle at which the ramp of the ramp mechanism extends relative to the brake disk can change over the course of the ramp. A special case or limit case of a ramp mechanism is a wedge mechanism in which the angle at which the ramp extends to the brake disk is constant over the course of the ramp. In this case, the ramp is called a wedge. A plurality of ramp mechanisms may be provided, which are distributed over a back side of the friction brake lining facing away from the brake disk and that brace the friction brake lining jointly.
In the known friction brake, balls or rollers are provided as roller bodies, by way of which the friction brake lining is braced on the ramp and which reduce friction upon displacement of the friction brake lining along the ramp. The roller bodies in the known friction brake roll on raceways that are provided on a back side, facing away from the brake disk, of the friction brake lining and on an abutment plate facing toward the friction brake lining. The raceways extend in the displacement direction of the friction brake lining and are at their lowest point at the center of their length; their depth decreases from the center of the length in both directions. As a result, the raceways form the ramp or ramps of the ramp mechanism. The raceways are mounted in sunken fashion in the back side of the friction brake lining and in the abutment plate; they guide the roller bodies, and via the roller bodies the friction brake lining, transversely to the displacement direction.
The self-boosting electromechanical friction brake has been described above in terms of a disk brake, because it can be explained clearly in terms of a disk brake since known friction brakes of this kind are predominantly embodied as disk brakes, and because even the friction brake named as prior art is a disk brake. This does not preclude the implementation of the invention in other types of brake.
If the friction brake lining of the known friction brake is pushed back and forth for braking and for releasing the brake, the roller bodies ideally roll along the raceways, and they are in their original position when the friction brake lining is also in its original position again. This is not true if in a self-boosting friction brake, to compensate for wear, the friction brake lining is not restored entirely. The “wandering away” or creeping described below of the roller bodies occurs nevertheless. It cannot be precluded that the roller bodies will not merely roll but will also slide on their raceways when the friction brake lining is pushed back and forth. In that case, the roller bodies no longer return to their original position. When the friction brake lining has been pushed back and forth many times, the roller bodies can creep farther and farther from their outset position.